We propose to determine the general mechanisms whereby the substrate chain length influences the rate of hydrolysis by exonucleases. The influence of protein factors important in DNA synthesis on the non-productive binding of DNA polymerases to interior substrate residues will be studied. We wish to define a set of inhibitors of DNA replication that interrupt synthesis at specific steps. Toward this goal, the mechanism of inhibition of DNA synthesis by nalidixic acid and novobiocin will be explored by a combined genetic and enzymological approach. Drug-resitant mutants will be sought that are altered in the target proteins as will strains that overproduce these proteins. The target proteins for nalidixic acid and novobiocin will be purified on the basis of in vitro complementation, characterized physically and enzymologically, and their role in DNA replication and the nature of their interaction with the drugs pursued.